Intervertebral implant with keel

ABSTRACT

An intervertebral implant component of an intervertebral implant includes an outer surface for engaging an adjacent vertebra and an inner surface. A keel extends from the outer surface and is designed to be disposed in a slot provided in the adjacent vertebra. This keel extends in a plane which is non-perpendicular to the outer surface; and preferably there are two of the keels extending from the outer surface which are preferably offset laterally from one another. In another embodiment, an anterior shelf is provided at an anterior end of the outer surface, and this anterior shelf extends vertically away from the inner surface in order to help prevent bone growth from the adjacent vertebra towards the inner surface. Further in accordance with disclosed embodiments, various materials, shapes and forms of construction of the component and/or keel provide various benefits.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 15/888,505 filed Feb. 5, 2018, which is acontinuation application of U.S. patent application Ser. No. 14/643,287filed Mar. 10, 2015, now U.S. Pat. No. 9,387,086, which is acontinuation application of U.S. patent application Ser. No. 12/375,071filed Nov. 20, 2009, now U.S. Pat. No. 8,998,990, which is a nationalstage application of International Patent Application No.PCT/US2007/074218 filed Jul. 24, 2007, which claims priority to U.S.Provisional No. 60/832,595 filed Jul. 24, 2006 and entitled “NEXTGENERATION PRODISC C IMPLANT DESIGN: IP DISCLOSURE,” the contents ofwhich are incorporated in their entirety by reference.

BACKGROUND OF THE INVENTION

Historically, when it was necessary to completely remove a disc frombetween adjacent vertebrae, the conventional procedure was to fuse theadjacent vertebrae together. This “spinal fusion” procedure, which isstill in use today, is a widely accepted surgical treatment forsymptomatic lumbar and cervical degenerative disc disease.

More recently, there have been important developments in the field ofdisc replacement, namely disc arthroplasty, which involves the insertionof an artificial intervertebral disc implant into the intervertebralspace between adjacent vertebrae. Such a disc implant allows limiteduniversal movement of the adjacent vertebrae with respect to each other.The aim of total disc replacement is to remove pain generation (causedby a degenerated disc), restore anatomy (disc height), and maintainmobility in the functional spinal unit so that the spine remains in anadapted sagittal balance. Sagittal balance is defined as the equilibriumof the trunk with the legs and pelvis to maintain harmonious sagittalcurves and thus the damping effect of the spine. In contrast with fusiontechniques, total disc replacement preserves mobility in the motionsegment and mimics physiologic conditions.

One such intervertebral implant includes an upper part that cancommunicate with an adjacent vertebrae, a lower part that cancommunicate with an adjacent vertebrae, and an insert located betweenthese two parts. To provide an anchor to the adjacent vertebrae, eachpart includes a vertically extending keel. Examples of this type ofimplant are disclosed in U.S. Pat. No. 5,314,477 (Marnay) and U.S. Pat.No. 7,204,852 (Marnay et al.), which are hereby incorporated byreference.

While this and other known implants represent improvements in the art ofartificial intervertebral implants, there exists a continuing need forimprovements for these types of implants.

It will also be noted that in order to provide a keel slot in avertebra, a cutting of the bone needs to be performed. Typically the cutis made by chiseling, drilling or milling. Combinations of theseprocedures are possible too. However, where a chisel cut is made using achisel and a mallet, quite high forces are applied in direction of thecut. With drilling, lesser forces are applied, but the drill can slip ofor bend during drilling. With milling, a precise cut is made withouthigh forces, but the milling tool needs to have a certain diameter,because otherwise it will brake during milling so milling is not alwayspossible where a long narrow cut is required. Thus, a procedure used toperform narrow cuts without applying high forces is desirable. Exemplaryof such prior art devices and methods are those disclosed in USPA2004-0215198 (Marnay et al.) and USPA 2006-0064100 Bertagnoli et al.),which are hereby incorporated by reference.

BRIEF SUMMARY OF THE INVENTION

In accordance with the a disclosed embodiment, an intervertebral implantincludes two components each having an outer surface for engaging anadjacent vertebra and an inner surface. A keel extends from the outersurface of one component and is designed to be disposed in a slotprovided in the adjacent vertebra. This keel extends in a plane which isperpendicular to the outer surface. In one preferred embodiment, thereare a pair of keels extending from the other outer surface, which arepreferably offset laterally from one another. The pair of keels arepreferably symmetrically located on either side of a vertical mid-planeof the outer surface, and are divergent or convergent with respect toeach other.

Also in accordance with a disclosed embodiment, an intervertebralimplant component of an intervertebral implant includes an outer surfacefor engaging an adjacent vertebra and an inner surface. A keel extendsfrom the outer surface and is designed to be disposed in a slot providedin the adjacent vertebra. An anterior shelf is also provided at ananterior end of the outer surface, and this anterior shelf extendsvertically away from the inner surface in order to help prevent bonegrowth from the adjacent vertebra towards the inner surface. Inaccordance with preferred embodiments, the anterior shelf can have aforward surface which is angled, an exterior surface which is polished,and/or a surface treatment which helps prevent bone growth thereon.

Further in accordance with disclosed embodiments, various materials andforms of construction of the component are disclosed. Posterior and/oranterior reductions of the keel are possible for different benefits. Thebody strength of the vertebra with keel slots on both the superior andinferior surfaces can also be stronger if the keels of the associatedcomponents requiring slots are laterally offset. Embodiments ofcomponents with modular keels, as well as a variety of advantageous keelshapes (both in horizontal cross section and vertical cross section) arealso disclosed.

It will also be appreciated that various combinations of the featuresdisclosed hereafter for a component, and hence for the implant, are alsopossible as desired.

An instrument for cutting of keel slots with a saw blade, and inparticular for cutting multiple slots simultaneously, is also provided.

Other features and advantages of the present invention are stated in orapparent from detailed descriptions of presently preferred embodimentsof the inventions found hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bottom, right and back perspective view of an implantcomponent in accordance with the present invention.

FIG. 2 is a top and front perspective view of the implant componentdepicted in FIG. 1 .

FIG. 3 is cross-sectional side elevational perspective view of theimplant component of FIG. 1 taken along the line 3-3 of FIG. 1 .

FIG. 4 is an exploded front, bottom, left side perspective view of theimplant component of FIG. 1 .

FIG. 5 is a bottom and back perspective view of a second embodiment ofan implant component in accordance with the present invention.

FIG. 6 is a top, front and left side perspective view of the implantcomponent depicted in FIG. 5 .

FIG. 7 is cross-sectional side elevational perspective view of theimplant component of FIG. 5 taken along the line 7-7 of FIG. 5 .

FIG. 8 is an exploded bottom, right and back perspective view of theimplant component of FIG. 5 .

FIG. 9 is a top, front and left side perspective view of an implantcomponent with a pointed keel.

FIG. 10 is a top, back and right side perspective view of an implantcomponent with an H-shaped keel. FIGS. 4A-4D illustrate an exemplaryembodiment of another spine stabilization device, in which:

FIG. 11 is a top, back and left side perspective view of an implantcomponent inserted in a vertebra with an anterior corner of the keelexposed. FIG. 4B illustrates a partial cutaway view of the spinestabilization device of FIG. 4A;

FIG. 12 is a top, back and left side perspective view of an implantcomponent with a keel with a rounded top corner.

FIG. 13 is a top, back and left side perspective view of an implantcomponent with a keel with a chamfered top corner.

FIG. 14 is a top, back and left side view of a vertebra withsymmetrically cut slots for centered keels.

FIG. 15 is a top and front perspective view of mating implant componentswith two keels of one component offset from the single keel of the othercomponent.

FIG. 16 is top, back and left side view of a vertebra with offset cutslots for the keels of the components depicted in FIG. 15 .

FIG. 17 a front view of mating implant components with one keel of onecomponent offset from the other keel of the other component.

FIG. 18 is a front and top perspective view of an implant component withtwo keels that are divergent.

FIG. 19 is a front and top perspective view of an implant component withtwo keels that are convergent.

FIG. 20 is a left side view of an implant component having a bone growthretarding plate which is inserted in a vertebra.

FIG. 21 is a top plan view of the implant component of FIG. 20 .

FIGS. 22-24 are top plan views of implant components with differentmodular keel shapes retainable therein.

FIGS. 25-31 are top, back and left side perspective views of implantcomponents with differently shaped keels for better longitudinalretention.

FIGS. 32-51 are front elevation views of different keel shapes.

FIGS. 52-58 are top plan views of different keel shapes.

FIGS. 59-61 are schematic front elevation views of mating implantcomponents with different thicknesses of keels.

FIG. 62 is a top, back and right side perspective view of an instrumentused to cut keel slots.

FIG. 63 is a top, front and right side perspective view of the cuttingtool depicted in FIG. 62 .

FIG. 64 is a top, front and right side perspective view of analternative cutting tool for use with the instrument depicted in FIG. 62.

DETAILED DESCRIPTION OF THE INVENTION Materials

With reference now to the drawings in which like numerals represent likeelements throughout the views, a first component 10 of an intervertebralimplant for total disc replacement according to the present invention isdepicted in FIGS. 1-4 . The implant including component 10 is primarilydesigned for insertion between adjacent vertebrae from an anteriordirection. Thus, reference will sometimes be made to anterior andposterior directions for convenience. However, it will be appreciatedthat insertion from other directions is possible, and hence thereferenced directions would thus be similarly changed. In addition,terms such as front/back, forward/rearward, left/right and top/bottommay be used to identify directions as depicted in the figures and/or asthe implant is used relative to any insertion direction, even though the“front” may be facing anteriorly or posteriorly depending on thedirection of insertion used. Thus, these terms are used for illustrationpurposes only and not as limiting terms for the invention.

In this embodiment, implant component 10 is formed of an endplate 12having an outer surface 14 and an integral keel 16 extending outwardlyaway from outer surface 14. Outer surface 14 is designed to engage anadjacent vertebra, with integral keel 16 then being located in a slotsuitably formed in the vertebra (see FIG. 11 for an illustration of avertebra having a component mounted therein). Component 10 also includesan inner surface 18 in which an insert 20 is securely located. Insert 20includes in this embodiment a concavity 22 therein, but it will beappreciated that insert 20 could instead have a convexity. Received inconcavity 22 will be a mating component of the implant, allowing a balljoint movement of a similar endplate engaging an adjacent vertebra. Thesimilar endplate could have a mating convexity provided thereon, eitherintegrally formed or as an insert (like insert 20); or alternatively thesimilar endplate could be part of a substantially identical component,and a third component could be interposed between the two components toprovide articulation between the facing concavities (or convexities) ofthe components.

Component 10 is designed to help overcome the problem of artifacts whicharises when an MRI is taken of a metal orthopedic medical device such asan intervertebral implant typically having two such components. Duringspine surgery, MRI is a standard diagnostic tool used to determine thestate of the anatomy by visualizing the soft tissue and nerve rootsrelative to the bony anatomy. However, commonly used metals fororthopedic devices cause MRI artifacts of different degrees. The amountof imaging artifact is reduced as the density of the material andmagnetic properties of the material are decreased. For example, thefollowing biomedical materials create imaging artifacts in decreasingorder: stainless steel, CoCr alloy, Titanium alloy, ceramics and plasticpolymer materials.

The design of component 10 is made to have a reduced amount of imagingartifact, and is thus comprised of two materials: a soft low densitymaterial with properties similar to that of the surrounding bone as themain construct foundation, and a harder dense material insert withsuperior wear properties for the articulating surface areas or parts. Inparticular, component 10 includes endplate 12 made of a titanium ortitanium alloy, and insert 20 made of a material with good articulatingproperties such as Co—Cr. It will also be noted that insert 20 isreduced to a small area defining the articulating surface of theimplant, further helping to reduce the MRI artifact problem.

Conveniently, insert 20 is a shrink-fit into a cylindrical portion 24 ofa receiving and mating cavity 26 of endplate 12 located adjacent innersurface 18, though other ways to secure insert 20 as known in the artare possible (such as a sliding/locking ledge design, threads,adhesives, soldering, pressing, etc.). In this embodiment, it will beappreciated that the receiving cavity 26 of endplate 12 has beenoptionally designed to pass through outer surface 18, so that a portionof insert 20 is viewable and flat with outer surface 18 as shown in FIG.2 .

Depicted in FIGS. 5-8 is an alternative embodiment of a component 10′which is broadly similar to component 10 and whose similar elements willthus be identified with the same reference numerals followed by a prime(′). In this embodiment of component 10′, insert 20′ has a wide flatbottom 28 (as shown best in FIG. 7 , for example) which mates with thesimilar shape of cavity 26′. This bottom 28 is used to secure insert 20′to endplate 12′ by use of a suitable adhesive or the like, while thecylindrical areas 30 on the sides of insert 20′ in cavity 26′ areslightly spaced from the surrounding metal of cylindrical portion 24′ ofendplate 12′ to create an air channel 25 between the two as shown. Itwill also be noted that cavity 26′ does not extend through to outersurface 14′, as shown in FIG. 6 .

Other alternatives to the embodiments above could include any number ofhard materials and/or surface treatments to be used for the articulatingfunction, such as a ceramic insert, a titanium nitride coated hardenedsurface, a diamond coated surface such as a DLC (amorphic diamond likecarbon), or any other type surface treatment or material for medical usethat provides a hardened superior wear surface. A layer of Co—Cr,ceramic, carbon or other biocompatible low-friction material could alsobe plasma coated and/or sputtered onto the low-density material of anendplate in a position thereof providing the articulating area (balland/or socket). This layer of material can then be ground, polished,and/or treated to create the desired low-friction, low-wear articulatingsurfaces.

Alternatives to the titanium base could be PEEK, PEKK, or some otherstructural polymer, carbon reinforced or other similar compositematerials, or any other low density structural biomaterial. A possiblealternative could be a pyrolitic carbon implant with a smootharticulating surface and roughened bone contacting surfaces similar tothat used in hand and wrist implants.

Posterior Keel Reduction

It has been found that during surgical implantation final seating of theimplant endplates has in a few cases proven to be difficult. It isbelieved that during surgical preparation (typically chiseling, orcutting or drilling) of the keel receiving channel in the vertebralbody, not all of the cut bone material is removed but instead somematerial may be forced to the posterior (or closed) end of the channelby the action of the chisel or the like. This material is theninadvertently left to form an obstruction to the full seating of thekeel at the closed end of the channel, resulting in a suboptimal implantposition. In order to alleviate this problem, a number of designs areproposed with means designed to accommodate for such excess material, asby a posterior (forward) reduction of the keel. The concept is to createa significantly reduced angled/inclined surface to the forward (orposterior) edge of the keel, more pronounced than the large chamfer atthe forward end of the keel such as shown in U.S. Pat. No. 7,204,852which is designed instead for easier insertion of the keel.

Thus, a means for accommodating excess material in accordance with thepresent invention is shown in FIG. 9 . In this embodiment, component 36has a keel 38 with a front edge formed as a pointed edge 40 (or knifeedge shape). With pointed edge 40, keel 38 can easily cleave through anyexcess material located in the bottom end of the channel in thevertebra. A similar solution would be to reduce the length of the keelby increasing the distance between the posterior (forward) face of thekeel and the posterior (bottom) edge of the component. The depth(bottom) of the clearance cut preparation in the bone of the channel forthe keel would then remain the same, but the anterior to posterior (ortrailing to leading) length of the keel would be reduced on theposterior (forward or leading) end. Doing this would create addedclearance between the bone material left in the channel and theposterior (forward) surface of the keel. FIG. 3G shows a bottomperspective view of the spine stabilization device of FIG. 3E. Theinferior plate 26 may also include an opening within which resides abottom cap 50. This bottom 50 cap may also contain an opening, as shown.This bottom cap 50 may be configured with a curved, round, or chamferededge so that the inferior plate closely matches the lower endplate ofthe intervertebral space. Like the other domed cap 40, this domed cap 50may be eccentric or centric.

Still another means for accommodating excess material is shown in FIG.10 . In this embodiment, component 44 has a keel 46 with a forwardU-shape, and preferably an overall an H-shape in plan view. This U-shapecan be provided by removing remove material out of the central forwardportion of keel 46 so as to create a slot 48 that goes down the front ofkeel 46. Any obstructive bone material in the channel would fill intothe slot 48 and allow keel 46 to fully seat in the formed channel.

Anterior Keel Reduction

It has also been found that in rare cases, due to the irregular anatomyof some patients and/or in cases of extreme anterior (rearward)positioning of the implant, an anterior (rearward) top (outer) corner 52of a keel may be proud or protruding out from the anterior (rearward)surface 54 of the bone as shown in FIG. 11 . This exposed metal cornercan cause irritation in surrounding tissue. For that reason, a means forslightly reducing the anterior (rearward) corner profile in the keel inthe anterior (rearward) end (as well as posterior end, if desired, asnoted above) is desirable.

One means for slightly reducing the anterior (rearward) corner profilein the keel at the anterior end is to provide a component 58 with a keel60 in which the anterior (rearward) corner is reduced by a curvedrounded surface 62 as shown in FIG. 12 . Another means for slightlyreducing the anterior (rearward) corner profile in the keel at theanterior (rearward) end is to provide a component 64 with a keel 66 inwhich the anterior (rearward) corner is reduced by a chamfer 68 as shownin FIG. 13 .

Other embodiments could include an angled surface or other feature toreduce keel material in this area.

Vertebral Body Strength

In small or weakened vertebrae, weakening of the bone due to thealignment in the central axis of the spine of keel receiving channels 74and 76 superior and inferior to vertebra 72 in a multi-level(consecutive) disc replacement case will occur as shown in FIG. 14 .This occurrence would leave a much smaller central portion of the bonein vertebra 72, possibly creating a weaker bony construct surroundingthe two keels of the two adjacent implants. Thus, it was determined thatdesigns where the keels would be offset in different locations would bedesirable for leaving a more intact stronger bone construct.

One means for offsetting keels in adjacent implants is depicted in FIGS.15-16 . In this embodiment, an implant 80 has a single central keel 80on one (superior) endplate 82 and double laterally offset keels 84 onthe other (inferior) endplate 86. Obviously, the positions of doublekeels could instead be superior, if desired. However, the desired resultof a stronger vertebra is as shown in FIG. 16 where such an implant isused both above and below the depicted vertebra. In particular, thereare double keel receiving channels 90 in the superior side of vertebra92 which are on either side (laterally offset) from single keelreceiving channel 94 in the inferior side of vertebra 92—leaving alarger amount of bone in the central area of vertebra 92.

As a variation of this design, the two components of an implant couldhave oppositely offset keels 98 a and 98 b as depicted in FIG. 17 .

Another variations of this design could include embodiments whereinoffset keels 84′ are divergent at a certain angle as shown in FIG. 18 ,or offset keels 84″ are convergent as shown in FIG. 19 . The enhancedbenefit of having divergent/convergent keel designs is to preventloosening of the associated endplate with that type of geometry. Thedivergent or convergent angles of the keels would add greater resistanceto movement in the axial direction and become less likely to be loosenedover time. Thus, it will be appreciated that such divergent orconvergent keel designs would be beneficial even when not used inconsecutive implants; and thus the keels of paired endplates could beboth convergent or both divergent, or one convergent and one divergent.

Prevention of Fusion

It has been found that in some cases of total disc arthroplasty in theneck, bone has grown across the implant located in between the vertebralbodies so that the adjacent vertebrae have become fused in spite of thearticulating implant provided therebetween. Mostly this problem occursin the anterior portion of the implant. Therefore, in order to preventthis from happening, a means is added to the implant endplates to retardor stop bone from bridging over the implant.

Depicted in FIGS. 20-21 is an implant component 100 having an endplate102 and insert 104. Provided at the anterior (outer) end of component102 is a raised plate edge or shelf-like feature 106. The presence ofraised edge 106 serves as a means for retarding bone growth/formationbetween the adjacent vertebrae by its presence and large distance whichmust then be bridged.

It will also be appreciated that tissue, including bone tissue, tends togrow into and anchor to rough surfaces of titanium implants but does notadhere to certain plastics or other materials. Thus, this raisedanterior edge 106 of endplate 104 is preferably polished to a smoothsurface finish. Alternatively, the raised anterior edge 106 is treatedwith a suitable surface coating since bone fusion is usually related tothe blood supply and cell formation/cell growth for a given area. Forexample, an anti-cellular coating could be placed in this area toprevent bone forming and hence undesirable bone fusion. Alternatively,an anti-blood coagulating surface or agent could be integral to raisededge 106. Raised edge 106 could also be designed to hold cement or othermaterial that would contain an anti-coagulant or anti-cellular growthinhibiting agent. The bone cement, implant coating, and/or implant edgefusion block feature could also contain a controlled releaseanti-inflammatory agent to retard the healing process and thus retardbone growth in that area of undesired fusion.

Modular Keels

In order to improve fixation of a keel while avoiding removing of someof the vertebra, the keel or other fixation feature of an implant couldalso be modular as depicted in FIGS. 22-24 . With such an embodiment,the endplate is placed into the intervertebral space first; and then thefixation element, such as a keel, is moved through the plate into amating slot provided in the bone. Finally, that fixation element is thenlocked to the plate by a suitable mechanical means such as a fastener orset screw. Thus, depicted in FIG. 22 is an implant component 110 havingan endplate 112 in which there is a central aperture 114 shaped tomatingly receive a keel 116. As shown, set screw 118 is used to securekeel 116 in place in aperture 114 of endplate 112 once keel 116 islocated properly. Keel 116 has a “ball” shape at one end, with thevertebra thus having a cutout or slot designed to receive this ballshape. With this shape, keel 116 is more stably held in placelongitudinally, without removing so much of the vertebrae.

Other possible keel geometries or configurations are possible, such asthe “Xmas Tree” shape of keel 116′ of component 110′ depicted in FIG. 23, or the snake-shaped keel 116″ of component 110″ depicted in FIG. 24 .It would also be alternatively possible to put the keel in place first,and then attach the endplate to the keel in a similar manner.

Keels Shapes

A variety of different keel shapes are also possible to enhance theability of the keel to be retained (or not to loosen as easily) in theslot cut into the vertebra. For example, the shape of the keel can beslightly wedged in one dimension (in the forward or insertion direction)as shown by keel 122 in FIG. 25 (or keel 126 in FIG. 27 ); or in twodimensions as shown by keel 124 in FIG. 26 . Typically the wedge shapeis regular and symmetrical, but it could also be irregular andunsymmetrical. The wedge shape can go over the whole length of the keelor just in a short section of the keel. The wedge shape of the keel doesnot need to be solid, and thus could be hollow as shown by keel 126 inFIG. 27 . In this embodiment, keel 126 is open from the top, and fromthe anterior (outer) end; and in another embodiment the keel could alsobe open from the front (forward end) as shown schematically by keels 128in FIG. 28 . Keel 126 or 128 could also be open from the sides asdesired; and it will be thus appreciated that combinations of differentopenings are also possible.

A keel with steps is also desirable, as shown by keels 130, 132 and 134shown in respective FIGS. 29, 30 and 31 . In the case of keel 130, itconsists of respective small parts 130′ that are grouped together tobuild the keel-shape. It will be appreciated that keel 134 also providesspecial surfaces preventing the backing out by the keel, namely angledout fin or surface 134′ and angled in surface 134″, even though keel 134is easy to insert in one (forward) direction as surface 134′ has aramping action in that direction.

While the vertical cross-sectional shape of a keel is typically a simplerectangular shape, the vertical cross-sectional geometry of a keel couldalso be modified to enhance fixation and/or stability in the bone. Thiscross section can vary, as it can be symmetrical or asymmetrical. Someexamples of different vertical cross-sectional shapes, and combinationsof vertical cross-sectional shapes, are shown in the drawings asdescribed hereafter with reference to the noted figures.

FIG. 32 : bowed on each side;

FIG. 33 : angled asymmetrically on each top side;

FIG. 34 : pointed at the top side;

FIG. 35 : angled to one side;

FIG. 36 : top edge angled;

FIG. 37 : right angle triangle shaped;

FIG. 38 : round nosed;

FIG. 39 : one side inwardly angled;

FIG. 40 : one side inwardly bowed;

FIG. 41 : both sides inwardly bowed;

FIG. 42 : top edge angled to one side which is angled inward;

FIG. 43 : angled on each top side, and each side angled inward;

FIG. 44 : both sides angled inward;

FIG. 45 : both sides angled outward;

FIG. 46 : diamond shaped;

FIG. 47 : both ends chamfered;

FIG. 48 : parallelogram shaped;

FIG. 49 : elongated hexagonal shaped;

FIG. 50 : both ends stepped at both sides; and

FIG. 51 : elongated pentagon shaped.

Also, while the horizontal cross-sectional shape of a keel is alsotypically a simple rectangular shape, the horizontal cross-sectionalgeometry of a keel could also be modified to enhance fixation and/orstability in the bone. This cross section can vary, as it can besymmetrical or asymmetrical. Some examples of different horizontalcross-sectional shapes, besides those already mentioned above, andcombinations of horizontal cross-sectional shapes, are shown in thedrawings as described hereafter with reference to the following figures.

FIG. 52 : angled symmetrically toward the leading edge;

FIG. 53 : angled symmetrically toward the leading edge to a point;

FIG. 54 : rounded leading edge;

FIG. 55 : sharply angled to one side leading edge;

FIG. 56 : a series of angled leading edges;

FIG. 57 : diamond shaped with trailing edge blunted; and

FIG. 58 : asymmetrically bowed.

Further, where more than one keel is present on one of the twocomponents the thicknesses of the keels can vary. Some examples ofdiffering thickness keels are shown in the drawings as describedhereafter with reference to the following figures.

FIG. 59 : the top keels have a small and a large thickness, while thebottom keel has an intermediate thickness;

FIG. 60 : the bottom keels have a small and an intermediate thickness,while the top keel has a large thickness;

FIG. 61 : the top keels have a small and a large thickness, while thebottom keels have a small intermediate thickness and a largeintermediate thickness.

Cutting of Dual Keel Slots

As noted above, instruments and methods have been disclosed for cuttingkeel receiving slots in a vertebra, or in two adjacent vertebrae.Typical of such devices and methods are those shown and described inUSPA 2004-0215198 (Marnay et al.) and USPA 2006-0064100 Bertagnoli etal.) which primarily disclose chiseling or burring embodiments.

Depicted in FIGS. 62-63 is an instrument broadly similar to thosedisclosed in the above noted published applications, and thus includinga trial implant 150 having an adjustable stop 152. The trial implant 150includes a top slot 154 at the location above which a keel slot is to becut in a superior (or inferior) vertebra when trial implant 150 islocated between two vertebrae. It will be noted that trial implant alsohas two bottom slots (not shown) similar to top slot 150, but atlocations where offset keel slots are to be cut in the inferiorvertebra—so that trial implant 150 is thus used to cut the slots for animplant such as disclosed in FIG. 15 .

Extending away from trial implant 150 is a guide 156 which is used toguide trial implant into the intervertebral space between two adjacentvertebrae after the disc is removed and to which adjustable stop 152 isthreadedly engaged. Guide 156 has slots corresponding to those in trialimplant 150, such as top slot 158. Slots 154 and 158 serve to guide sawcutting tool 160 therealong, where cutting tool 160 is rapidlyreciprocated by a suitable motor 162 shown schematically and which cantake the form of various power tools as known in the art. Rapidreciprocation of saw cutting tool 160 is effective to produce is areduced impact on the vertebral bone due to the acceleration to massrelationship between cutting tool 160 and the vertebral bone.

It will be appreciated that cutting tool 160 includes three thin sawblades 162 which extend at a proximal edge thereof along slots in guide156 and trial implant 150, such as slots 158 and 154. At the distaledge, saw blades 162 have suitable cutting teeth 164, which at a leadingor forward end form a ramp for easier starting into the vertebra. Theinsertion depth of cutting blades into the vertebrae is controlled byadjusting the position of adjustable stop 152.

While cutting tool 160 has been shown with three blades, it will beappreciated that only a single blade could be provided to cut each slotindividually as needed. It would also be possible to provide a blademore like a chisel but with cutting teeth just at the front. Differentinterchangeable blades would also be possible, if a narrower or wider,or higher or lower, or deeper or shallower, cut slot was desired. Ifdesired, motor 162 can be dispensed with, and the blade or blades movedby hand with the same guidance. The material of the blades is preferablya suitable metal, but ceramic or plastic blades, or even a diamondcutting blade, would also be possible. If desired or necessary, acoatings to reduce friction could be used with the cutting blades.

Depicted in FIG. 64 is another cutting tool 168 usable in place ofcutting tool 160 and with trial implant 150. Cutting tool 168 includeschisel blades 170 and would thus be used to chisel three slotssimultaneously.

Various advantageous features have been described above with respect tovarious embodiments. Such advantageous features are also considered tobe usable together, rather than singly as typically depicted anddescribed.

While the present invention has been described with respect to exemplaryembodiments thereof, it will be understood by those of ordinary skill inthe art that variations and modifications can be effected within thescope and spirit of the invention.

What is claimed is:
 1. An intervertebral implant for insertion into anintervertebral disc space between two adjacent vertebrae along aninsertion direction, the implant comprising: a component having an outersurface for engaging one of the adjacent vertebrae and an inner surfaceopposite the outer surface, the component further including first andsecond fixation elements extending from the outer surface, said firstand second fixation elements extending in a plane which is substantiallyperpendicular to said outer surface, wherein each of the fixationelements has a leading end in the insertion direction and a trailing endopposite the insertion direction, wherein the first fixation elementextends from the leading end to the trailing end in a first directiontransverse to the insertion direction and wherein the second fixationelement extends from the leading end to the trailing end in a seconddirection transverse to the insertion direction and transverse to thefirst direction.
 2. The intervertebral implant of claim 1, wherein theleading end of the second fixation element is spaced from the leadingend of the second fixation element relative to the insertion direction.3. The intervertebral implant of claim 1, wherein the componentcomprises an anterior portion, a posterior portion and at least twosidewalls connecting the anterior portion to the posterior portion,wherein the component defines an axis from the anterior portion to theposterior portion and the leading and trailing ends of the first andsecond fixation elements are disposed lateral to the axis.
 4. Theintervertebral implant of claim 3, wherein the leading edge of the firstand second fixation elements are disposed lateral to the axis and thetrailing ends of the first and second fixation elements are disposedlateral of the axis on an opposite side of the axis such that each ofthe first and second fixation elements each extend across the axis. 5.The intervertebral implant of claim 1, wherein the first and secondfixation elements include opposed side walls that are substantiallyparallel to each other.
 6. The intervertebral implant of claim 1,further comprising a second component having an outer surface forengaging the other of the two adjacent vertebrae and an inner surfaceopposite the outer surface, and third and fourth fixation elementsextending from the outer surface, wherein the third and fourth fixationelements on the second component each have a leading end in theinsertion direction and a trailing end opposite the insertion direction,wherein the third fixation element extends from the leading end to thetrailing end in a third direction transverse to the insertion directionand wherein the fourth fixation element extends from the leading end tothe trailing end in a fourth direction transverse to the insertiondirection and transverse to the third direction.
 7. The intervertebralimplant of claim 1, wherein the first and second fixation elements aresubstantially aligned with each other in the insertion direction todefine a keel.
 8. The intervertebral implant of claim 7, furtherincluding an insert residing between the first and second components. 9.An intervertebral implant for insertion into an intervertebral discspace between two adjacent vertebrae along an insertion direction, theimplant comprising: a component having an outer surface for engaging oneof the adjacent vertebrae and an inner surface opposite the outersurface, the component comprising an anterior portion, a posteriorportion and at least two sidewalls connecting the anterior portion tothe posterior portion, wherein the component defines a longitudinal axisfrom the anterior portion to the posterior portion, the componentfurther including at least first and second fixation elements extendingfrom the outer surface, said fixation elements extending in a planewhich is substantially perpendicular to said outer surface, wherein eachof the fixation elements has a leading end in the insertion directionand a trailing end opposite the insertion direction, wherein the leadingend of the first fixation element is disposed on a first side of thelongitudinal axis and the leading end of the second fixation element isdisposed on a second side of the axis opposite the first side, whereinthe trailing end of the first fixation element is disposed on the secondside of the longitudinal axis and the trailing end of the secondfixation element is disposed on the first side of the longitudinal axis.10. The intervertebral implant of claim 9, wherein the first and secondfixation elements extend across the axis from the leading end to thetrailing end.
 11. The intervertebral implant of claim 9, wherein thesecond fixation element is spaced from the first fixation element in theinsertion direction.
 12. The intervertebral implant of claim 9, whereinthe first and second fixation elements include opposed side walls thatare substantially parallel to each other.
 13. The intervertebral implantof claim 9, further comprising at least a third fixation element havinga leading end and a trailing end, wherein the leading end of the thirdfixation element is disposed on the one side of the axis and thetrailing end of the third fixation element is disposed on the oppositeside of the axis.
 14. The intervertebral implant of claim 9, wherein thefirst and second fixation elements are substantially aligned with eachother in the insertion direction to define a keel.
 15. Theintervertebral implant of claim 9, further comprising a second componenthaving an outer surface for engaging the other of the two adjacentvertebrae and an inner surface opposite the outer surface and comprisingan anterior portion, a posterior portion and at least two sidewallsconnecting the anterior portion to the posterior portion, wherein thecomponent defines an axis from the anterior portion to the posteriorportion, and third and fourth fixation elements extending from the outersurface, wherein each of the third and fourth fixation elements of thesecond component has a leading end in the insertion direction and atrailing end opposite the insertion direction, wherein the leading endof the third fixation element is disposed on one side of the axis andthe leading end of the fourth fixation element is disposed on anopposite side of the axis.
 16. The intervertebral implant of claim 15,further including an insert residing between the first and secondcomponents.